Transfer feature tracking to CommonRuby

Document in appropriate places that CommonRuby is the place to file and track feature changes. Examples of such places: bugs.ruby-lang.org top-level page, pages for the existing "trunk" and version-specific Redmine projects, ruby-lang.org pages that describe how to file bugs.

Decide how to handle the "feature" flag in the current trackers. This may include solutions like removing it entirely (perhaps infeasible if existing features are not migrated to CommonRuby), adding a warning or error when "feature" is selected to encourage users to file under CommonRuby instead, or automatically putting "feature" issues under CommonRuby at submission time.